Colin,
If your bottom line pursuit is to find a gel refill for a Sierra, you could check out the following Parker-Style:
Schmidt Super Bowl 8900
Visconti Gel Ballpoint - medium (.7)
Standard Parker brand Gel refill
I am currently researching for the best-in-class gel point refill in the Parker-style. The Schmidt Super Bowl has a nice line width without being "drippy" or prone to smudging, and is a very solid black. The Visconti is a slightly narrower line, but handles nicely. These two are #1 and #2 in my testing so far for color and handling.
A new refill by Schmidt called the Easyflow 9000M is almost like a hybrid between a gel and a regular ballpoint. It has more of a ballpoint feel, but looks like a gel ink! Impressive blackness.